1. Field of the Invention
The present invention relates to Mobile IP network technology. More particularly, the present invention relates to methods and apparatus for enabling Home Agent redundancy.
2. Description of the Related Art
Mobile IP is a protocol which allows laptop computers or other mobile computer units (referred to as “Mobile Nodes” herein) to roam between various sub-networks at various locations—while maintaining internet and/or WAN connectivity. Without Mobile IP or related protocol, a Mobile Node would be unable to stay connected while roaming through various sub-networks. This is because the IP address required for any node to communicate over the internet is location specific. Each IP address has a field that specifies the particular sub-network on which the node resides. If a user desires to take a computer which is normally attached to one node and roam with it so that it passes through different sub-networks, it cannot use its home base IP address. As a result, a business person traveling across the country cannot merely roam with his or her computer across geographically disparate network segments or wireless nodes while remaining connected over the internet. This is not an acceptable state-of-affairs in the age of portable computational devices.
To address this problem, the Mobile IP protocol has been developed and implemented. An implementation of Mobile IP is described in RFC 2002 of theIP Routing for Wireless/Mobile Hosts Working Group, C. Perkins, Ed., October 1996. Mobile IP is also described in the text “Mobile IP Unplugged” by J. Solomon, Prentice Hall. Both of these references are incorporated herein by reference in their entireties and for all purposes.
The Mobile IP process and environment are illustrated in FIG. 1A. As shown there, a Mobile IP environment 2 includes the internet (or a WAN) 4 over which a Mobile Node 6 can communicate remotely via mediation by a Home Agent 8 and a Foreign Agent 10. Typically, the Home Agent and Foreign Agent are routers or other network connection devices performing appropriate Mobile IP functions as implemented by software, hardware, and/or firmware. A particular Mobile Node (e.g., a laptop computer) plugged into its home network segment connects with the internet through its designated Home Agent. When the Mobile Node roams, it communicates via the internet through an available Foreign Agent. Presumably, there are many Foreign Agents available at geographically disparate locations to allow wide spread internet connection via the Mobile IP protocol. Note that it is also possible for the Mobile Node to register directly with its Home Agent.
As shown in FIG. 1A, Mobile Node 6 normally resides on (or is “based at”) a network segment 12 which allows its network entities to communicate over the internet 4 through Home Agent 8 (an appropriately configured router denoted R2). Note that Home Agent 8 need not directly connect to the internet. For example, as shown in FIG. 1A, it may be connected through another router (a router R1 in this case). Router R1 may, in turn, connect one or more other routers (e.g., a router R3) with the internet.
Now, suppose that Mobile Node 6 is removed from its home base network segment 12 and roams to a remote network segment 14. Network segment 14 may include various other nodes such as a PC 16. The nodes on network segment 14 communicate with the internet through a router which doubles as Foreign Agent 10. Mobile Node 6 may identify Foreign Agent 10 through various agent solicitations and agent advertisements which form part of the Mobile IP protocol. When Mobile Node 6 engages with network segment 14, it composes a registration request for the Home Agent 8 to bind the Mobile Node's current location with its home location. Foreign Agent 10 then relays the registration request to Home Agent 8 (as indicated by the dotted line “Registration”). During the registration process, the Home Agent and the Mobile Node 6 may then negotiate the conditions of the Mobile Node's attachment to Foreign Agent 10. For example, the Mobile Node 6 may request a registration lifetime of 5 hours, but the Home Agent 8 may grant only a 3 hour period. Therefore, the attachment may be limited to a period of time. When the negotiation is successfully completed, Home Agent 8 updates an internal “mobility binding table” which links the Mobile Node's current location via its care-of address (e.g., a collocated care-of address or the Foreign Agent's IP address) to the identity (e.g., home address) of Mobile Node 6. Further, if the Mobile Node 6 registered via a Foreign Agent, the Foreign Agent 10 updates an internal “visitor table” which specifies the Mobile Node address, Home Agent address, etc. In effect, the Mobile Node's home base IP address (associated with segment 12) has been binded to the care-of address such as the Foreign Agent's IP address (associated with segment 14).
Now, suppose that Mobile Node 6 wishes to send a message to a Corresponding Node 18 from its new location. An output message from the Mobile Node is then packetized and forwarded through Foreign Agent 10 over the internet 4 to Corresponding Node 18 (as indicated by the dotted line “packet from MN”) according to a standard internet protocol. If Corresponding Node 18 wishes to send a message to Mobile Node—whether in reply to a message from the Mobile Node or for any other reason—it addresses that message to the IP address of Mobile Node 6 on sub-network 12. The packets of that message are then forwarded over the internet 4 and to router R1 and ultimately to Home Agent 8 as indicated by the dotted line (“packet to MN(1)”). From its mobility binding table, Home Agent 8 recognizes that Mobile Node 6 is no longer attached to network segment 12. It then encapsulates the packets from Corresponding Node 18 (which are addressed to Mobile Node 6 on network segment 12) according to a Mobile IP protocol and forwards these encapsulated packets to a “care of” address for Mobile Node 6 as shown by the dotted line (“packet to MN(2)”). The care-of address may be, for example, the IP address of Foreign Agent 10. Foreign Agent 10 then strips the encapsulation and forwards the message to Mobile Node 6 on sub-network 14. The packet forwarding mechanism implemented by the Home and Foreign Agents is often referred to as “tunneling.”
FIG. 1B illustrates a significant problem with the Mobile IP system 2. If Home Agent 8 fails or otherwise become inoperative (due to a power failure, rebooting, scheduled maintenance, etc.), Mobile Node 6 is left without the ability to (1) receive new internet messages addressed to it at network segment 12 and (2) register with other Foreign Agents. In effect, Mobile Node 6 is cut-off from internet connection when Home Agent 8 goes down. This problem may extend to other Mobile Nodes supported by Home Agent 8. Often, a given Home Agent will be responsible for servicing numerous Mobile Nodes which may be based at sub-network 12, and hence, all registered Mobile Nodes will lose connectivity.
As shown in FIG. 1B, when Home Agent 8 fails, not only is network segment 12 disconnected from the internet, but Mobile Nodes at remote locations are also blocked from the registration and packet receipt functions of Mobile IP. In some networks, there may be other routers connecting segment 12 to the internet. Such additional routers would allow fixed hosts on the segment to maintain their internet connections but would not allow remote access to Mobile Nodes. Similarly, if Foreign Agent 10 should fail, all Mobile Nodes visiting sub-network 14 lose connections, even though there may be other routers on that sub-network.
A redundancy protocol known as Hot Standby Router Protocol (“HSRP”) is widely used to back up primary routers for a network segment. In HSRP, a “standby” or “secondary” router is designated as the back-up to an “active” or “primary” router. The standby router is linked to the network segment or segments serviced by the active router. The active and standby routers share a “virtual IP address” and possibly a “virtual Media Access Control (MAC) address.” All internet communication to and from the relevant sub-network employs the virtual IP and MAC addresses. At any given time, the active router is the only router adopting the virtual addresses. Then, if the active router should cease operation for any reason, the standby router immediately takes over its load (by adopting the virtual addresses). Further details of HSRP can be found in RFC 2281, “Cisco Hot Standby Router Protocol (HSRP)” by T. Li, B. Cole, P. Morton, and D. Li and in U.S. Pat. No. 5,473,599 issued to Li and Cole on Dec. 5, 1995. Both of these references are incorporated herein by reference in their entireties and for all purposes.
If Home Agent 8 participated in a HSRP (together with other routers connected to segment 12), its failure would allow those nodes currently plugged into sub-network 12 to maintain their communications with internet 4. However, its failure would leave Mobile Node 6 stranded on network segment 14. HSRP has no mechanism for handling internet communications via Mobile IP. This is partly due to the fact that in Mobile IP it is not enough to simply have a standby router ready to take over as active router. The Home Agent (active router) must carry-out higher level functions required by Mobile IP such as keeping track of the locations (and associated Foreign Agents) of the various Mobile Nodes for which it is responsible. Similarly, a Foreign Agent must keep track of visiting Mobile Nodes and their associated Home Agents.
One redundancy mechanism for Mobile IP has been proposed. It goes by the acronym HARP which stands for Home Agent Redundancy Protocol. It was presented in an Internet Engineering Task Force memo of Chambless and Binkley entitled “Home Agent Redundancy Protocol” and having a URL of ftp://ietf.org/internet-drafts/draft-chambless-mobileip-harp-00.txt. This protocol provides for a redundant or “peer” Home Agent which is intended to contain a record of the Mobile Node locations stored in the primary Home Agent. While this proposed protocol does provide a redundancy mechanism for Mobile IP Home Agents, it has certain short comings. Notably, it does not make use of a widely installed redundancy protocol. Therefore, to implement HARP, many enterprises must undertake a rather significant change to its existing network solution. More importantly, HARP is concerned only with Mobile IP. Many other network functions such as Network Address Translation (“NAT”), IP security, Reflexive Access List, etc. all could profit from router redundancy. However, each of these has its own specific high level requirements (analogous to the mobility binding table required for Mobile IP). HARP cannot be easily extended to these Non-Mobile IP network functions.
Unlike HARP, HSRP is a widely installed redundancy protocol. However, even if HSRP were implemented as the underlying redundancy protocol to implement Home Agent redundancy in a Mobile IP environment, there are several disadvantages associated with HSRP. First, HSRP enables only one router for a given network segment to act as the standby router at a single point in time. In other words, a redundancy scheme implemented using HSRP could include only a single backup Home Agent. Second, HSRP must be implemented on the same Local Area Network (LAN). It would therefore be beneficial if Home Agent redundancy could be implemented to enable multiple Home Agents to act as a backup for an active router. Moreover, it would be desirable if such a scheme could be implemented in a Wide Area Network (WAN) as well as a LAN. In addition, it would be beneficial if such a Home Agent redundancy scheme could be implemented regardless of the underlying routing redundancy protocol used to support such a scheme.
For the above reasons, an improved Home Agent redundancy protocol is required for Mobile IP.